Growth Regulation and Residue Safety of 6-Benzyladenine in Vietnamese Balm (Elsholtzia ciliata (Thunb.) Hyl.): A Stage-Specific and Data-Driven Assessment

Elsholtzia ciliata is an aromatic medicinal species valued for its essential oils, yet strategies to optimize biomass production, leaf development, and biochemical performance remain underexplored. Current knowledge lacks an integrated evaluation of how exogenous 6-benzyladenine (6-BA) modulates vegetative morphology, photosynthetic physiology, and consumer safety across horticultural time scales. In this study, growth dynamics were quantified over eight weeks, and controlled foliar 6-BA applications were performed at two developmental stages (week 4 and week 6), followed by high-resolution phenotyping, biochemical assays, and residue analysis. Machine-learning models (XGBoost, random forest, elastic net, linear regression) were trained to predict growth and evaluate feature contributions through SHAP analysis. The results showed significant time-dependent increases in vegetative biomass, with fresh biomass rising from 0.7 to 40.2 g plant⁻¹ between week 1 and week 8. Exogenous 6-BA significantly enhanced leaf number, leaf area, and biomass, with optimal responses at 15 ppm and week-4 spraying. XGBoost achieved superior predictive accuracy (R² up to 0.94), revealing dose, timing, and post-spray duration as key determinants. Residue and dietary risk assessments indicated rapid 6-BA dissipation and negligible chronic risk across age groups. These findings establish a quantitative framework for growth stage–specific regulation of E. ciliata and support 6-BA as a safe and effective horticultural enhancer. Future studies should incorporate multi-environment validations and metabolomic profiling to improve translational applicability in commercial cultivation systems.